1. Field of the Invention
The present invention relates to a voltage non-linear resistor consisting essentially of zinc oxide, and a method of producing the same.
2. Related Art Statement
Heretofore, voltage non-linear resistors consisting essentially of zinc oxide have superior non-linear voltage-current characteristic properties, so that they are widely used as surge absorbers for surge absorption and lightning arrestor for voltage stabilization. The voltage non-linear resistors are produced by adding a small amount of bismuth oxide, antimony oxide, cobalt oxide, or manganese oxide for exhibiting a voltage non-linear property to the main component zinc oxide, mixing, granulating, and forming the admixture to form a formed body, sintering the formed body preferably after an application of an inorganic matter for forming a side highly resistive layer, and attaching electrodes to the sintered bodies.
For using voltage non-linear resistors thus obtained as lightning arrestor for absorbing a large surge, the voltage non-linear resistors desirably have a large discharge current withstanding capability. The discharge current withstanding capability can be expressed by a value of a maximum electric current that does not incur breakage or surface flash-over when an impulse electric current of a wave form of 4/10 .mu.s is applied twice at an interval of 5 minutes and the electric current value is raised stepwise until the breakage or surface flash-over occurs.
Discharge current withstanding capability of the voltage non-linear resistor is considered to depend on voids or pores in the sintered body. Namely, the breakage at the time when the impulse electric current of the wave form of 4/10 .mu.s is applied is considered to be due to thermal stress, so that an improvement of discharge current withstanding capability can be expected, if the voids are reduced and a mechanical strength of the sintered body is enlarged. In addition, if the voids are present in the sintered body at the time of passing an electric current therethrough, the electric current is concentrated at distal ends of the voids cross passing to the direction of the electric current. If the concentration occurs within a short period, such as 4/10 .mu.s, heat conduction to the ambient is so small that a local temperature rise of the sintered body takes place. The local temperature rise generates a thermal stress which leads to breakage of the sintered body if the thermal stress exceeds a mechanical strength of the sintered body. Therefore, desirably the mechanical strength of the sintered body is enhanced, while the voids are removed in order to prevent concentration of the electric current at the distal ends of the voids. A method of obtaining a sintered body not having the voids is disclosed in Japanese Patent Application Laid-open No. 58-28,802, wherein temperature raise of a formed body from 800.degree. C. to 1,150.degree. C. during the temperature raising step of the sintering process is effected in a reduced pressure of not exceeding the atmospheric pressure.
However, the method of the Japanese Patent Application Laid-open No. 58-28,802 discloses merely an improvement of discharge current withstanding capability evaluated by an electric current of a rectangular wave form of 2 ms as regard to an effect of the decrease of the voids (to be referred to as "switching surge current withstanding capability", hereinafter), and nothing about discharge current withstanding capability evaluated by an impulse electric current of a wave form of 4/10 .mu.s (to be referred to as "lightning discharge current withstanding capability", hereinafter). Switching surge current withstanding capability and lightning discharge current withstanding capability are originally different from each other in nature, as seen in breakage forms of penetration breakage of the former and burst breakage of the latter. Therefore, the voids are considered to have different influence on switching surge current withstanding capability from lightning discharge current withstanding capability. The "penetration breakage" used herein means a breakage of forming a penetration hole of a diameter of about 1 mm in the voltage non-linear resistor and decreasing the resistance of the resistor to 1 K1/3 or less to lose the non-linear voltage-current characteristic property of the voltage non-linear resistor. The "burst breakage" used herein means a breakage of forming a crack in the voltage non-linear resistor or bursting the resistor into pieces. As described above, the burst breakage is caused by the thermal stress generated at the time of applying a lightning discharge current on the voltage non-linear resistor.
In addition, the method of the Japanese Patent Application Laid-open No. 58-28,802 conducts the heating to 1,150.degree. C. in the sintering process in a reduced pressure, i.e., in a low oxygen partial pressure state, so that oxidation of the formed body begins for the first time after the heating temperature exceeded 1,150.degree. C. in the temperature-raising step of the sintering process. Therefore, if the formed body to be sintered has some large size in diameter and thickness, such as a diameter of 40 mm and a thickness of 20 mm, a holding at the sintering temperature for a few hours can not sufficiently oxidize the interior of the formed body, so that the non-linear voltage-current characteristic property comparable to that of the ordinary product sintered in the atmosphere can not be obtained, though the voids are decreased. Moreover, if the holding time of the formed body at the sintering temperature is prolonged in order to oxidize the interior of the formed body, Bi.sub.2 O.sub.3 component is evaporated during the sintering process, so that a nonhomogeneous sintered body is merely obtained.
Furthermore, usual overvoltage protective apparatuses, such as a lightning arrestor insulator and the like, have to provide a side highly resistive layer on a side surface of the voltage non-linear resistor, in order to prevent a surface flash-over. The side highly resistive layer is usually formed by applying an inorganic matter on a side surface of a formed body to be sintered, and reacting the inorganic matter with the side surface of the formed body by sintering, so that it has a good coherent property to the sintered body. Thus, the inorganic matter applied on the side surface of the formed body should not peel off from the side surface, even when the formed body is shrunk by the sintering. In this respect, in the method of the aforementioned Japanese Patent Application Laid-open No. 58-28,802, the formed body shrinks rapidly at a temperature of around 850.degree. C., so that a large difference of shrink is caused between the inorganic matter and the formed body to peel off the former from the latter. Thus, the method has a drawback in that a side highly resistive layer of a good coherent property and a homogeneous property can not be formed on a side surface of the voltage non-linear resistor.